1. Field of the Invention
The present invention relates to an image forming apparatus for forming and bearing an unfixed toner image on a material to be recorded with a transfer system or a direct system like electrofax by an appropriate image forming process such as an electrophotographic process, electrostatic recording process and a magnetic recording process and fixing the unfixed toner image on the material to be recorded by heat-fixing means.
2. Related Background Art
As fixing means in an image forming apparatus using an electrophotographic process, a fixing device of a film heating system has been put to practical use in response to a demand for energy saving. In the fixing device of the film heating system, a thin film having heat resistance is nipped between a ceramic heater as heat generating means and a pressure roller as a pressuring member to form a fixing nip. The fixing device applies heat and pressure to a material to be recorded while conveying the material to be recorded by rotating both the film and the pressure roller, thus fixing the unfixed toner image thereon.
In this fixing device of the film heating system, since a heat capacity of a film functioning as a member to be heated is extremely small compared with a conventional fixing device of a heat roller system, thermal energy from the heat generating means can be used efficiently in a fixing process. Consequently, a quick start is possible which can reduce a waiting time from the input of a power supply of the image forming apparatus to a printable state. Moreover, since the heat capacity of the member to be heated is small and it is unnecessary to preheat the member to be heated during print standby, power consumption of the image forming apparatus can be controlled to be low and energy saving is possible.
As a higher efficiency fixing device of a film heating system, there is proposed a fixing device of an electromagnetic induction heating system for causing a conductive film itself to generate heat. Japanese Utility Model Application Laid-Open No. 51-109739 discloses a fixing device for inducing an eddy current to a metal film by an alternating magnetic field and causing the metal film to generate heat by Joule heat. Since it is possible to cause the film itself to generate heat in the electromagnetic induction heating system, thermal energy from heat generating means can be used more efficiently in a fixing process.
Such a fixing device of the film heating system by a heater or the film heating system by electromagnetic induction can be warmed up to a fixable temperature in sixty seconds or less and is called a quick start fixing device.
When it is attempted to reduce a time from the start of printing until the end of discharge of a printed transferring material (first printout time; hereinafter referred to as FPOT) making use of the characteristics of the quick start fixing device as described above, it is necessary to anticipate a temperature rising state of the fixing device and perform image formation in parallel with the temperature rising of the fixing device such that the transferring material reaches the fixing device when the temperature rising of the fixing device is completed.
However, in such a case, for example, where an image forming apparatus is placed under a low temperature environment or where a voltage of a power supply to the image forming apparatus has dropped to a lower limit value, temperature of the fixing device may not be able to completely rise in an assumed time. Thus, if timing is set with a margin of time from the start of printing until the start of fixing taking into account a worst condition, the FPOT is prolonged even under a normal condition.